2. Cover and bring to a gentle boil over medium-high heat, then reduce heat to low and simmer uncovered for 4 to 5 minutes.

3. Remove from heat (the mint should still be vibrant green; remove from the heat before the color dulls) and let it sit for 10 minutes.

4. Pour through a strainer and discard solids.

5. Meanwhile, put remaining ginger and watermelon in a blender and puree until well blended.

6. Pour the mixture through a strainer lined with a double layer of cheesecloth and let stand for 15 minutes until all the liquid has dripped through. Do not press the puree; doing so will make the juice cloudy.

7. Mix the mint syrup and watermelon-ginger juice in a sealed glass jar and chill in the fridge. Syrup will stay fresh for up to 3 days.

8. To serve, pour 1/2 cup syrup into a tall glass, top with 6 to 8 ounces chilled seltzer or soda water, and stir.